Electrochemical energy storage or generation devices include batteries, capacitors and ultracapacitors, and fuel cells. Batteries are characterized by rated capacity measured by ampere-hours that can be delivered. Capacitors and ultracapacitors are characterized by the energy or power density that can be delivered in a single discharge, which is the ratio of cell energy or power to weight or volume.
All energy storage or generation devices include a pair of electrodes and an electrolyte that is disposed between the electrodes to conduct an electric current. Electrodes are electrical conductors at the surface of which a change occurs from conduction by electrons to conduction by ions. A cathode electrode contains an active material that is reduced during discharge, whereas an anode electrode contains an active material that is oxidized during discharge. Conventional electrodes, particularly cathodes, are characterized by low electrical conductivity, dependent upon the type of device and materials used. For instance, transition metal-based lithium oxide electrodes for lithium-ion batteries and amorphous transition metal-based oxide electrodes for ultracapacitors both suffer from low cathode conductivity.
To compensate for low electrode conductivity, conductive fillers, such as carbon, are frequently added to the matrix to improve the conductivity. The requirement for and the amount of carbon filler can depend on the specific oxide used and the volume of filler can be as high as forty to fifty percent of the overall cathode mixture, dependent upon relative carbon density and other cathode components. Carbon fillers also require a binder, which can further reduce the conductivity and thus decrease the specific energy of the storage device.
As the filler concentration increases, the carbon particles can clump and aggregate and uniform dispersion becomes problematic. Non-uniform dispersion can compromise both electrode performance and lifetime by decreasing the percentage of contact surface area exposed to the electrolyte. As a result, existing approaches to improving energy storage device performance generally focus on electrode construction.
U.S. Pat. No. 6,616,875 to Lee et al., issued Sep. 9, 2003, discloses a manufacturing method for a metal oxide electrode for an ultracapacitor. An amorphous manganese oxide electrode is formed by absorbing potassium permanganate onto a conductive material, such as carbon or activated carbon, and is mixed with a solution to form amorphous manganese oxide. Conductive carbon is used as the active material and requires a binder to compensate for the low electrical conductivity of the manganese oxide.
U.S. Pat. No. 6,703,163 to Ogura et al., issued Mar. 9, 2004, discloses a lithium battery and electrode. A plurality of carbon nanotubes are dispersed in a conductive matrix containing an electrically conductive polymer and an organic compound having a disulfide group that is responsible for electrochemical reactions at the electrode. The carbon nanotubes conduct electricity along an axial direction of the matrix to decrease electrical resistance and improve conductivity over the electrically conductive polymer. However, the carbon nanotubes also function as a filler and require a binder.
U.S. Pat. No. 6,395,427 to Sheme et al., issued May 28, 2002, discloses a negative active material for and a method of preparing a rechargeable lithium battery. The negative active material includes a crystalline carbon core and a semi-crystalline carbon shell. Amorphous or crystalline carbon is mixed into a catalyst element and agglomerated to form carbon core particles that can provide micro-porous channels to improved electrolyte immersion. However, the carbon core particles function as a filler and can suffer from non-uniform diffusion.
Accordingly, there is a need for preparing and providing an energy storage device electrode with increased electrical conductivity and improved power density without relying on carbon with a binder in the matrix.